Such a trench transistor and a method for producing it is described, for example, in DE 102 34 996 A1.
In such components, the gate electrode is formed adjacently to a body zone and dielectrically insulated from this body zone. The gate electrode is used in a familiar manner for forming a conductive channel in the body zone when a suitable driving potential is applied, which body zone lies between a source zone and a drift zone which forms a part of the drain zone.
The field electrode, which can be at a different potential from the gate electrode, for example at source potential, is arranged adjacently to the drift zone and dielectrically insulated from the drift zone. If the component is blocking, the field electrode is used for compensating for charge carriers in the drift zone which are the result of the doping of the drift zone, in order to increase by this means the dielectric strength of the component in the blocking case. This compensation effect of field electrodes is described, for example, in U.S. Pat. No. 4,941,026, U.S. Pat. No. 5,973,360 or U.S. Pat. No. 5,283,201.
To ensure that a conductive channel will form in the body zone when a suitable driving potential is applied to the gate electrode, it is required that the gate electrode overlaps the source zone and the drift zone or that the gate electrode at least ends precisely on the boundary between body zone and source zone and the boundary between body zone and drift zone. In this arrangement, the gate-source capacitance formed between the source zone and the gate electrode and the gate-drain capacitance formed between the drift zone and the gate electrode are increased as the overlap increases, which has a negative effect on the switching speed of the component. If, in contrast, the gate electrode does not reach the boundary between body zone and source zone or body zone and drift zone, respectively, starting from the body zone, the aforementioned gate capacitances are reduced but the turn-on resistance is increased with a given driving potential or, respectively, the driving potential must increase to values higher than the starting voltage in order to still form a conductive channel. In the extreme case, when the distance between the end of the body zone and the gate electrode is too large, the formation of a conductive channel is completely prevented.
The distance between the gate electrode and the boundaries of the body zone to the adjacent source and drift zones can be adjusted via process parameters during the production of the component. Production processes for semiconductor components are unavoidably subjected to fluctuations which must be taken into consideration during the design of the component. Thus, for example during the production of a trench MOSFET, the relative position of the lower end of the gate electrode can fluctuate by a production-related tolerance with respect to the boundary between body zone and drift zone. To prevent the creation of a component in which the gate electrode does not reach the boundary between body zone and drift zone, the components are dimensioned in such a manner that gate electrode overlaps the drift zone at least by the dimension of this tolerance. However, this overlap leads to an increased gate-drain capacitance in components in which the maximum process tolerances are not reached.
A dimension figure for MOSFETs which is to be optimized with regard to switching losses and switching speed is the surface-independent product, i.e. the product related to the transistor surface, of the turn-on resistance Ron and the gate-drain capacitance. This dimension figure is also called the “Figure of Merit” (FOM).